The objective of this project is to understand the function of 5-methylcytosine in the DNA of animal cells. Special attention will be given to investigation of a possible role in oncogenesis and differentiation. The specific goals are: (1) to purify DNA methylase to homogeneity from a pure source, the mouse mast cell tumor P815Y; and (2) to identify mutants of P815Y which are defective in nucleic acid methylation.